Introduction

Glucagon is a peptide hormone of 29 amino acids produced and secreted by the A-cells of the pancreatic islets (Unger & Orci, 1981). The hormone is one of a family of several peptides with similar primary structures that includes secretin (Mutt et al, 1970), vasoactive intestinal peptide (Mutt & Said, 1974), gastric inhibitory peptide (Brown, 1971), and growth hormone releasing hormone (Spiess et al, 1982). The secretion of glucagon is regulated by blood levels of glucose (Gerich et al, 1974) and amino acids (Assan et al., 1977), as well as by a variety of hormonal stimuli (Samols et al., 1983). The action of glucagon on its target tissues, particularly the liver, is an important factor in protein and carbohydrate metabolism (Aoki et al., 1974; Cherrington et al., 1976). Abnormal regulation of glucagon gene expression has been implicated in the pathogenesis of diabetes mellitus (Dobbs et al, 1975).

Peptides related immunologically to glucagon are produced in several extrapancreatic tissues such as brain (Tager et al., 1980), salivary glands (Lawrence et al, 1977) and intestine (Conlon, 1980; Parker et al, 1984; Hoshino et al, 1984). The principal hormonal function of glucagon is to regulate carbohydrate, fat and protein metabolism, but it is also possible that glucagon and glucagon-related peptides function as paracrine agents by way of communicating with adjacent cells, for example within pancreatic islet cells and as neurotransmitters within the nervous system. Glucagon and glucagon-related peptides have been isolated and characterized from catfish (Andrews & Ronner, 1985) and anglerfish (Andrews et al, 1986) endocrine pancreases.